Field of the Invention
The invention concerns a method and device for driving a capacitive control element, especially a piezoelectrically operated fuel injection valve of an internal combustion engine.
Piezo control elements consist of a number of piezoceramic layers and form a so-called "stack", which changes its dimensions when a voltage is applied, especially its length s by a deviation ds, or generates an electrical voltage during mechanical compression or tension.
The electrical properties of this type of piezostack vary with the temperature to which it is exposed. With increasing temperature, its capacitance is increased, but so is the deviation. At the temperatures to be considered for automotive applications of about -40.degree. C. to +140.degree. C., changes of up to a factor of 2 are observed.
A drive circuit for a piezoelectric element of a matrix printer is known from U.S. Pat. No. 5,387,834, in which a temperature sensor senses the temperature of the piezoelectric element. Driving of the piezoelectric element occurs with charging times that are stored in a table as a function of the temperature.
The deviation varies roughly linearly with the applied voltage at a specified control element capacitance or a specified temperature. If the temperature varies, so does the deviation at equal voltage. On the other hand, the deviation varies in proportion to the square of the applied energy (ds.about.e.sup.2), but independently of the temperature. It is very complicated to supply a specified amount of energy to a control element.